1. Field of the Invention
The present invention relates to a method of measuring the nuclear magnetic resonance in selected areas of a body. More specifically, the present invention relates to measuring the nuclear magnetic resonance in selected areas of a body to present images of body cross-sections according to a slice-selective two-dimensional Fourier-transformation method. Wherein the body is in a homogeneous magnetic field, exposed to a selection gradient and excited by a selective RF-pulse, and wherein a time-limited phase encoding gradient is applied, and at least one nuclear resonance signal by means of a read gradient is generated by gradient inversion in the form of at least one so-called gradient echo. The selection gradient, the phase encoding gradient and the read gradient are arranged orthogonally to one another. RF-pulses having a high frequency excitation profile, of which the base frequencies of the high frequency (slice selection frequency) differ by a value .DELTA.f corresponding to the distance between the centres of adjacent layers or slices, in such a manner that different slices are excited by RF-pulses in cooperation with the selection gradient, wherein the amplitude and/or duration of the phase encoding gradient is varied.
2. Description of Related Art
In the case of conventional methods of NMR tomography a number of m individual steps containing n complex data points in each case are necessary for receiving an image of the matrix size m.times.n. Depending on the type of spatial encoding, the image is then generated either by two-dimensional Fourier-transformation or (less frequently) by filtered back projection. In the case of two-dimensional imaging, m individual acquisition steps are necessary for encoding the spatial information in the second image direction. If the two-dimensional Fourier-transformation is used, the magnitude of the phase encoding gradient is varied at each individual step. In which case, the term "phase encoding step" is used. The time between two individual steps for each layer defines the contrast of the image. For a typical spin echo technique in clinical applications, a repeating interval of 500 ms or more has proven to be favorable. Within this repeating interval, individual steps from several layers or slices (typically around 10-20 layers or slices) can be acquired without the need for additional time (a so-called multi-slice technique). The image recording of a relatively large volume can be carried out where each phase encoding step is performed in all layers, then the next individual step is performed in all layers by changing the phase encoding gradient, and so forth until the recording of all phase encoding steps is completed. One individual step or phase encoding step comprises the sequence of RF-pulses used for the generation of the NMR signal, as well as, the sequence of time-variant magnetic field gradients required for slice selection and spatial encoding.
A disadvantage of this method is that the slices acquired in this manner cannot adjoin one another directly due to the use of selective (i.e. narrow-band) pulses for the slice selection. These RF-pulses, even for optimum pulse forms, do not have an ideal rectangular excitation profile. To prevent overlappings, a gap between two slice profiles is necessary, as shown as waveform "a" of FIG. 1. In FIG. 1, the excitation intensity is shown on the vertical axis, and the direction of the slice selection gradient (denoted z-direction) is shown on the horizontal axis. Owing to the excitation profiles which are not ideally rectangular, areas of little image intensity are generated in the areas between two successive slices. For the diagnosis of very small lesions (for example hypophyseal adenomae), the possibility that the lesion is not scanned with the given slice coverage cannot be ignored. Therefore, it is necessary to repeat the recording by displacing it by half the width of a slice, as shown as waveform "b" of FIG. 1. In this manner the examined volume is recorded completely; however, this doubles the measuring time.
A direct complete recording of a measuring volume is possible nowadays only by using three-dimensional recording techniques. These techniques, are time intensive when using spin echo sequences which provide a diagnostically favorable image contrast; however, such techniques are overly time intensive such that application thereof is not possible in practice. Other recordings may be carried out by using known rapid imaging sequences like FLASH or RARE (Magn. Reson. Med. 5,380-383 (1987), DE 34 34 161 C2, DE 35 04 734 A1). While these imaging sequences can provide a complete representation of a measuring volume within an acceptable time period, they frequently do not have the desired contrast.